(1) Field of the Invention
This invention relates to a foamable unsaturated polyester resin composition, an unsaturated polyester resin foam and a method of preparing such a foam.
(2) Description of the Prior Art
Unsaturated polyester resin is inexpensive and exhibits high strength characteristics and excellent resistance to heat. Therefore, this resin is commonly used for the materials for furniture, bath-tubs, marine structures, structural members or the like. In case unsaturated polyester resin having said excellent properties is able to be expanded, the foam provides a significant economic advantage in the value of the material saved by the weight reduction. In addition, the availability of acceptable low density polyester foams permits wide applications for polyester resins, by virtue of their good properties, i.e. lightness, thermal insulating properties and good sense of touch.
Heretofore, polyester foams have been prepared by the use of chemical blowing agent. However, the methods employed and the foams produced have not been wholly satisfactory or commercially successful. This is because the curing behavior of unsaturated polyester resin is different from that of other resins and makes it very difficult to produce low-density foams. Unsaturated polyester resin composition is normally liquid and exhibits fluidity, but with the proceeding of polymerization reaction the viscosity of said composition increases rapidly resulting in the gelation and thereafter the cure in no time. Therefore, the viscosity suitable for the foaming disappears in very short period of time, and even if the blowing agent is employed, it is very difficult to adjust the rate of decomposition of the blowing agent to the change in viscosity. In addition, the exothermic heat is small before gelation, but increases rapidly after gelation. Therefore, a volatile blowing agent can not be employed.
Notwithstanding said difficulties, the light-weight foamed product of unsaturated polyester resin has long been desired. Hence somewhat different approaches to overcoming said problems are proposed. One of them is disclosed in Japanese Patent Publication No. 39107/1975, which comprises adding expandable micro-capsules into unsaturated polyester resin composition and thereafter heating the mixture to expand the micro-capsuls and to cure the unsaturated polyester resin simultaneously thereby giving rise to a light-weight product. However, this method can not be applied to a cold molding, such as hand lay-up, spray-up and cold pressing commonly employed in the plastics industry.
There has been known a method for preparing a water-containing polyester, in which the polyester resin dispersed with fine particles of water is cured to obtain a foamed product. The product prepared by this method has a drawback that a water content is gradually lost from the surface of polyester resin with the passing of time, which results in the change in size and the warpage or distortion in the molded article.
One of the methods for producing unsaturated polyester resin foams by the use of blowing agent is disclosed in Japanese patent publication No. 29772/1973. The unsaturated polyester resin used in this method is not a usual solution of polyester dissolved in styrene monomer, but a solid or powder of prepolymer with diallyl phthalate. In this method, since the starting materials are solid, the molding must be carried out at an elevated temperature higher than the melting temperature of materials, e.g. above 100.degree. C. as described in said patent specification. Therefore, this method can not be applied to the cold molding.
Recently, there has been proposed in U.S. Pat. No. 3,920,590 a liquid polyester resin foamable composition from which such a foam is prepared. This composition comprises in combination a liquid unsaturated polyester resin, hydrogen peroxide or an organic peroxide as a curing agent, a cobalt compound as a promotor and a hydrazide compound as a blowing agent, both the peroxide-cuing agent and cobalt promotor being present in high concentration levels. In this system, the reactivity of organic peroxide with the hydrazide compound and cobalt compound is comparatively low, and thus the amount of gas evolved is rather little resulting in the lower expansion. Therefore, in said system the organic peroxide should be present in high concentration levels. However, in case the organic peroxide is used at high concentration, unsaturated polyester resin is cured very rapidly, and thus the expansion of polyester resin caused by the decomposition of blowing agent is suppressed due to said rapid curing, which results in the surface cracking or the lower expansion. Therefore, said patent literature teaches that the preferred curing agent is hydrogen peroxide.
Nevertheless, by the use of hydrogen peroxide as the curing agent in unsaturated polyester resin foamable composition, the following problems will arise. A commercially available hydrogen peroxide is an aqueous solution. Hence, in case an aqueous solution of hydrogen peroxide is added to the non-aqueous system or phase, particularly to the resin, the dispersion of hydrogen peroxide in the resin is not uniform and, as the result, the foamed product with uniform cellularity will not be obtained. In addition, hydrogen peroxide has the explosive reactivity and rapidly react with said other ingredients only by mixing them. Therefore, the preliminary mixing of hydrogen peroxide with other ingredients is not permitted, and the foamable composition is constituted by the multi-component system or multi-liquid system when used, which result in the difficulties of the operation and the controlling of reaction. Moreover, the presence of water in unsaturated polyester resin composition affects the curing of resin and will lead to the insufficient curing and the surface cracking. As being well known, hydrogen peroxide has per se the defect of poor stability. Furthermore, by the use of hydrogen peroxide, the foamed product is coloured to yellow.
As mentioned above, although unsaturated polyester foams have been produced, the techniques employed in preparing these foams having fine and uniform cellular structure are not commercially successful or practical.